Intramolecularly Hydrogen-Bonded Aromatic Pentamers as Modularly Tunable Macrocyclic Receptors for Selective Recognition of Metal Ions

Despite the tremendous progress that has been made in macrocyclic chemistry since the discovery of corands, cryptands, and spherands more than four decades ago, macrocyclic systems possessing a high level of controllability in structural configuration concurrent with a systematic tunability in function are still very rare. Employing an inner design strategy to orient H-bonding forces toward a macrocyclic cavity interior while convergently aligning exchangeable ion-binding building blocks that dictate a near-identical backbone curvature, we demonstrate here a novel pentagonal framework that not only enables its variable interior cavity to be maintained at near-planarity but also allows its ion-binding potential to be highly tunable. The H-bonded macrocyclic pentamers thus produced have allowed a systematic and combinatorial evolution of ion-selective pentamers for preferential recognition of Cs⁺, K⁺, or Ag⁺ ions

Interlayer Polymerization in Chemically Expanded Graphite for Preparation of Highly Conductive, Mechanically Strong Polymer Composites

The large-scale application of graphene–polymer composites needs a simple, low-cost method that simplifies the preparation process of graphene and optimizes the structure and properties of composites. We propose the first interlayer polymerization in chemically expanded graphite (CEG) with large specific surface areas, which allows CEG to be spontaneously exfoliated into single- and few-layer graphene in poly­(methyl methacrylate) (PMMA). Our results demonstrate that besides weakened interlayer interactions, the surface wettability of CEG to monomers is a critical prerequisite for the desired graphene exfoliation, dispersion, and performance optimization of composites. The slightly oxidized CEG (LCEG) improved to some extent the affinity for the monomer but is not sufficient to achieve complete exfoliation of LCEG, so that the resulting composites reveal the mechanical and electrical properties that are far poorer than those of the surface-modified LCEG-based composites. The latter not only exhibit a significantly enhanced elastic modulus, increased as much as 3-fold relative to that of the neat PMMA, but also show an extremely high electrical conductivity, of >1700 S/m. Such a novel interlayer polymerization approach is expected to accelerate the use of industrial applications of a wide range of graphene-based composites

Computational Insights into Processes Underlying the Amine-Induced Fluorescence Quenching of a Stimuli-Responsive Phenol-Based Hexameric Foldamer Host

Recently, we reported that amine-induced folding of a more fluorescent, more linear structure into less fluorescent, more curved or helically folded states enables patterned recognitions of amines and ammoniums. In this article, we have carried out extensive <i>ab initio</i> calculations at the B3LYP/6-31G level that not only map out the detailed amine-induced folding/quenching pathways and plausible folding/quenching species but also surprisingly reveal the binding of amines to anionic hosts to be unusually cooperative in a way that the progressively more charged anionic hosts act as increasingly better “amine trappers”. Accordingly amine-dependent folding occurs via a synergistic action of amines’ basicity and the progressively more curved backbone of the host. Although a hexamer carrying four deprotonable hydroxyl sites can reach a tetra-anionic state, mono-, di-, tri-, and tetra-anionic complexes likely dominate as the major quenching species in the presence of, respectively, 2, 4, 8, and 72 equiv of primary amines

Ultrafast Electron Transfer Kinetics in the LM Dimer of Bacterial Photosynthetic Reaction Center from <i>Rhodobacter sphaeroides</i>

It has become increasingly clear that dynamics plays a major role in the function of many protein systems. One system that has proven particularly facile for studying the effects of dynamics on protein-mediated chemistry is the bacterial photosynthetic reaction center from <i>Rhodobacter sphaeroides</i>. Previous experimental and computational analysis have suggested that the dynamics of the protein matrix surrounding the primary quinone acceptor, Q_A, may be particularly important in electron transfer involving this cofactor. One can substantially increase the flexibility of this region by removing one of the reaction center subunits, the H-subunit. Even with this large change in structure, photoinduced electron transfer to the quinone still takes place. To evaluate the effect of H-subunit removal on electron transfer to Q_A, we have compared the kinetics of electron transfer and associated spectral evolution for the LM dimer with that of the intact reaction center complex on picosecond to millisecond time scales. The transient absorption spectra associated with all measured electron transfer reactions are similar, with the exception of a broadening in the Q_X transition and a blue-shift in the Q_Y transition bands of the special pair of bacteriochlorophylls (P) in the LM dimer. The kinetics of the electron transfer reactions not involving quinones are unaffected. There is, however, a 4-fold decrease in the electron transfer rate from the reduced bacterio­pheophytin to Q_A in the LM dimer compared to the intact reaction center and a similar decrease in the recombination rate of the resulting charge-separated state (P⁺Q_A^–). These results are consistent with the concept that the removal of the H-subunit results in increased flexibility in the region around the quinone and an associated shift in the reorganization energy associated with charge separation and recombination

Folding-Promoted TBACl-Mediated Chemo- and Regioselective Demethylations of Methoxybenzene-Based Macrocyclic Pentamers

Tetrabutylammonium chloride (TBACl) salt alone has not been shown previously to be capable of removing methoxy groups. It is demonstrated here that the use of TBACl achieves efficient folding-promoted chemo- and regioselective demethylations, eliminating up to two out of five methyl groups situated in similar macrocyclic chemical microenvironments

Q‑Band Electron-Nuclear Double Resonance Reveals Out-of-Plane Hydrogen Bonds Stabilize an Anionic Ubisemiquinone in Cytochrome <i>bo</i>₃ from <i>Escherichia coli</i>

The respiratory cytochrome <i>bo</i>₃ ubiquinol oxidase from <i>Escherichia coli</i> has a high-affinity ubiquinone binding site that stabilizes the one-electron reduced ubisemiquinone (SQ_H), which is a transient intermediate during the electron-mediated reduction of O₂ to water. It is known that SQ_H is stabilized by two strong hydrogen bonds from R71 and D75 to ubiquinone carbonyl oxygen O1 and weak hydrogen bonds from H98 and Q101 to O4. In this work, SQ_H was investigated with orientation-selective Q-band (∼34 GHz) pulsed ¹H electron–nuclear double resonance (ENDOR) spectroscopy on fully deuterated cytochrome (cyt) <i>bo</i>₃ in a H₂O solvent so that only exchangeable protons contribute to the observed ENDOR spectra. Simulations of the experimental ENDOR spectra provided the principal values and directions of the hyperfine (hfi) tensors for the two strongly coupled H-bond protons (H1 and H2). For H1, the largest principal component of the proton anisotropic hfi tensor <i>T</i>_<i>z</i>′ = 11.8 MHz, whereas for H2, <i>T</i>_<i>z</i>′ = 8.6 MHz. Remarkably, the data show that the direction of the H1 H-bond is nearly perpendicular to the quinone plane (∼70° out of plane). The orientation of the second strong hydrogen bond, H2, is out of plane by ∼25°. Equilibrium molecular dynamics simulations on a membrane-embedded model of the cyt <i>bo</i>₃ Q_H site show that these H-bond orientations are plausible but do not distinguish which H-bond, from R71 or D75, is nearly perpendicular to the quinone ring. Density functional theory calculations support the idea that the distances and geometries of the H-bonds to the ubiquinone carbonyl oxygens, along with the measured proton anisotropic hfi couplings, are most compatible with an anionic (deprotonated) ubisemiquinone